The federal government does not require that employers reimburse for mileage. When employees pay for work related expenses, the employer has no obligation to pay them back. (There are exceptions like when expenses cause employees to fall below minimum wage.)
Except, that is, in California. Mileage reimbursement in California is required.
California law protects employees from bearing the burden of business expenses and so they must reimburse their employees when they drive.
California employers do not have to reimburse the IRS rate. They may instead reimburse actual expenses incurred. If they choose to do it this way, they must keep meticulous records of mileage and receipts. Using the IRS rate is generally easier, however.
From the California Labor Code:
2802. (a) An employer shall indemnify his or her employee for all necessary expenditures or losses incurred by the employee in direct consequence of the discharge of his or her duties, or of his or her obedience to the directions of the employer, even though unlawful, unless the employee, at the time of obeying the directions, believed them to be unlawful.
(b) All awards made by a court or by the Division of Labor
Standards Enforcement for reimbursement of necessary expenditures under this section shall carry interest at the same rate as judgments in civil actions. Interest shall accrue from the date on which the employee incurred the necessary expenditure or loss.
(c) For purposes of this section, the term “necessary expenditures or losses” shall include all reasonable costs, including, but not limited to, attorney’s fees incurred by the employee enforcing the rights granted by this section.